Recently new structural motifs that incorporate Fe-S clusters have been discovered, concomitant with the discovery of new functions for Fe-S enzymes. One of these functions is reduction of regulatory disulfides during light activation of chloroplast enzymes. The Fe-S enzyme responsible for this process, ferredoxin thioredoxin reductase, contains an [Fe4S4]2+ cluster which in the reaction mechanism is oxidized to [Fe4S4]3+ at an unusual redox potential (- 200 mV vs. NHE) when compared to the potential for the same redox couple in HIPIP. It was proposed that a five-coordinate iron site is responsible for the potential change. Synthetic [Fe4S4(LS3)(dithiolate)]3- clusters containing a five- coordinate iron site can be indeed reversibly oxidized at potentials between -750 mV and -400 mV( vs. SCE). The objective of this research plan is to synthesize, isolate, and characterize complexes containing [Fe4S4]3+ cores. Based on their eletrochemicalproperties, [Fe4S4(LS3)(dithiolate)]3- clusters are a rational choice for precursors of [Fe4S4(LS3)(dithiolate)]2- complexes formally containing [Fe4S4]3+ Cores. Two of the canonical forms which can be given for these complexes are [(Fe4S4)3+(LS3)3-(dithiolate)2-] and [(Fe4S4)2+(LS3)3-(semithione)--]. Dithiolates are non-innocent ligands and both forms are likely to determine the behavior of the complexes. In contrast, coordinated semiquinones and catechols appear in numerous metal complexes in charge-localized form. Thus use of catecholate as bidentate ligands may lead to oxidized clusters which can be represented as [(Fe4S4)2+(LS3)3-(semiquinone)--]. These complexes will represent the first models for a diamagnetic [Fe4S4]2+ core strongly interacting with a radical.